In any communication system using a bus common to a multiplicity of terminals or stations, the transmission and reception of messages is handled at each station in accordance with an established protocol. These protocols, as well as the means for their implementation, are the subject of international standards applicable to either point-to-point or hierarchically organized multipoint subsystems. A hierarchical multipoint subsystem assigns temporary roles of master and slave to associated stations.
One standardized procedure has become known as the Open Systems Interconnection (OSI) developed by the International Standards organization (ISO) as discussed in an article titled "Data Processing--Open Systems Interconnection--Basic Reference Model", published in the name of American National Standards Institute in Computer Networks 5 (1981), pages 81-118, by North-Holland Publishing Company. This model is based on the concept of a layered structure interposed between a user terminal and a communication medium or channel. Specifically, there are seven layers or levels each communicating with the adjoining layer or pair of layers of its own structure and with its peer in the corresponding structure of another station with which messages are being exchanged. The lower layers essentially conform to CCITT recommendation X.25. Further reference in this connection is made to an article by Harold C. Folts, titled "Coming of age: A long-awaited standard for heterogeneous nets", published in Data Communications of January 1981, pages 63-73.
Problems are encountered in adapting such a model to a system operating in the "broadcasting" mode in which a common channel accessible to a multiplicity of stations is designed to carry data packets pertaining to messsages exchanged between a given pair of stations intercommunicating in what may be termed a virtual point-to-point link. For the concurrent establishment of a certain number of such virtual links, the respective packets must be interleaved with rapid switchover between competing links. In the OSI model, the upper layers are implemented with the aid of microprocessors operating with a maximum clock frequency on the order of 10 MHz. With heavy traffic, as during peaks invariably occurring when a bus of bandwidth greater than 10 MHz is used for hybrid transmission (data, telephone, television and facsimile signals, for example), the processing time may become excessive.